Medical uses

A 2013 Cochrane overview and network meta-analysis concluded that varenicline is the most effective medication for tobacco cessation and that smokers were nearly three times more likely to quit on varenicline than with placebo treatment. Varenicline was more efficacious than bupropion or NRT and as effective as combination NRT for tobacco smoking cessation.[2][3]

The United States' Food and Drug Administration (US FDA) has approved the use of varenicline for up to twelve weeks. If smoking cessation has been achieved it may be continued for another twelve weeks.[4]

Varenicline has not been tested in those under 18 years old or pregnant women and therefore is not recommended for use by these groups. Varenicline is considered a class C pregnancy drug, as animal studies have shown no increased risk of congenital anomalies, however, no data from human studies is available.[5] An observational study is currently being conducted assessing for malformations related to varenicline exposure, but has no results yet.[6] An alternate drug is preferred for smoking cessation during breastfeeding due to lack of information and based on the animal studies on nicotine.[7]

Side effects

Mild nausea is the most common side effect and is seen in approximately 30% of people taking varenicline though this rarely (<3%) results in discontinuation of the medication.[3] Other less common side effects include headache, difficulty sleeping, and nightmares. Rare side effects reported by people taking varenicline compared to placebo include change in taste, vomiting, abdominal pain, flatulence, and constipation. In a recent meta-analysis paper by Leung et al., it has been estimated that for every five subjects taking varenicline at maintenance doses, there will be an event of nausea, and for every 24 and 35 treated subjects, there will be an event of constipation and flatulence respectively. Gastrointestinal side-effects lead to discontinuation of the drug in 2% to 8% of people using varenicline.[8][9] Incidence of nausea is dose-dependent: incidence of nausea was higher in people taking a larger dose (30%) versus placebo (10%) as compared to people taking a smaller dose (16%) versus placebo (11%).[10]

Depression and suicide

In November 2007, the US FDA announced it had received post-marketing reports of thoughts of suicide and occasional suicidal behavior, erratic behavior, and drowsiness among people using varenicline for smoking cessation. Since July 1, 2009, the US FDA has required varenicline to carry a black box warning that the drug should be stopped if any of these symptoms are experienced.[11] The label notes, however, that a pooled analysis of 18 randomized clinical trials including 8,521 people found similar rates of psychiatric events in the treatment and placebo arms, and that similar results have been obtained in four observational studies including 10,000 to 30,000 varenicline users.[12] People are advised to weigh the risks of using varenicline against the benefits of its use, noting that varenicline "has been demonstrated to increase the likelihood of abstinence from smoking for as long as one year compared to treatment with placebo." and that "the health benefits of quitting smoking are immediate and substantial."[12]

A 2014 systematic review did not find evidence of an increased suicide risk.[13] Other analyses have reached the same conclusion and found no increased risk of neuropsychiatric side effects with varenicline.[2][3]

Cardiovascular disease

In June 2011, the US FDA issued a safety announcement that varenicline may be associated with "a small, increased risk of certain cardiovascular adverse events in people who have cardiovascular disease."[14]

A prior 2011 review had found increased risk of cardiovascular events compared with placebo.[15] Expert commentary in the same journal raised doubts about the methodology of the review,[16][17] concerns which were echoed by the European Medicines Agency and subsequent reviews.[18][19] Of specific concern were "the low number of events seen, the types of events counted, the higher drop-out rate in people receiving placebo, the lack of information on the timing of events, and the exclusion of studies in which no-one had an event."

In contrast, multiple recent systematic reviews and meta-analyses have found no increase in overall or serious adverse cardiovascular events (including for individuals at risk of developing cardiovascular disease) associated with varenicline use.[19][20][21][22]

Post-marketing surveillance

No evidence for increased risks of cardiovascular events, depression, or self-harm with varenicline versus nicotine replacement therapy has been found in one post-marketing surveillance study.[23]

Varenicline's partial agonism on the α4β2 receptors rather than nicotine's full agonism produces less effect of dopamine release than nicotine's. This α4β2 competitive binding, reduces the ability of nicotine to bind and stimulate the mesolimbic dopamine system - similar to the method of action of buprenorphine in the treatment of opioid addiction.[3]

Pharmacokinetics

Most of the active compound is excreted by the kidneys (92–93%). A small proportion is glucuronidated, oxidated, N-formylated or conjugated to a hexose.[27] The elimination half-life is about 24 hours.

Varenicline received a "priority review" by the US FDA in February 2006, shortening the usual 10-month review period to 6 months because of its demonstrated effectiveness in clinical trials and perceived lack of safety issues.[33] The agency's approval of the drug came on May 11, 2006.[4] On August 1, 2006, varenicline was made available for sale in the United States and on September 29, 2006, was approved for sale in the European Union.[34]